1. Field of the Invention
The present invention relates to a projection display apparatus for magnifying and projecting optical images formed on light valves, and to an optical apparatus using the projection display apparatus.
2. Related Art of the Invention
A method for forming optical images on light valves depending on picture signals, for applying light to the optical images, and for magnifying and projecting the optical images on a screen via a projection lens is known as a method for obtaining large screen images. These days, attention is given to a projection display apparatus using a liquid crystal display as a light valve.
In addition, a method for using a reflecting light valve capable of increasing pixels without reducing the pixel aperture ratio of a liquid crystal panel has been proposed in U.S. Pat. No. 4,836,649 by Ledebuhr et al. to increase the resolution of projected images. Since it is not necessary to dispose a switching element between pixel electrodes in the case of the reflecting light valve, the pixel pitch can be made smaller, and a high density structure can be attained easily. Consequently, the brightness of the reflection type is higher than the transmission type, whereby projected images having high resolution can be obtained.
The basic structure of the reflecting light valve and its operation principle are described be low referring to FIG. 1. A photoconductive layer 3, an optically reflecting layer 4 and a liquid crystal layer 5 used as an optically modulating layer are disposed between two transparent electrodes 2, 6 formed on two glass substrates 1, 7. Voltage is applied across the two transparent electrodes 2, 6. Light 8 to be written from an image source enters the photoconductive layer 3 from the glass substrate 1. On the other hand, light 9 to be read enters the liquid crystal layer 5 from the glass substrate 7. The applied voltage changes depending on the image to be written formed on the photoconductive layer 3, whereby the light 9 to be read is modulated. The modulated light 9 to be read is reflected by the optically reflecting layer 4 and projected on a screen (not shown) as a projected image. A highly dielectric liquid crystal, a nematic liquid crystal or the like can be used as a material of the optically modulating layer. FIG. 2 shows a structure of a projection display apparatus using three reflecting light valves for red, green and blue colors, which is used to obtain a projected full-color image having high luminance and high resolution. Light comprising nearly parallel light beams enters a cold mirror 12 for transmitting ultraviolet radiation and infrared radiation and for reflecting visible light, and the light is separated into three primary color bands (green, blue and red light components) by a color separating optical system comprising dichroic mirrors 13, 14 and a plane mirror 15. The three primary color bands enter polarization beam splitters 19, 20, 21 corresponding to the color bands via plane mirrors 16, 17 and 18, respectively, and the color bands are each separated into an S-polarized light component which is reflected and a P-polarized light component which is transmitted. The S-polarized light components enter reflecting light valves 22, 23, 24 corresponding thereto as light components to be read. The reflecting light valves 22, 23, 24 are used to modulate the light components to be read using the double refraction characteristics of liquid crystals, and have such a structure as that shown in FIG. 1. The images of light components to be written from image sources 25, 26, 27, such as CRTs, are formed on photoconductive layers of the reflecting light valves 22, 23, 24 by writing lenses 28, 29, 30, respectively. The double refraction characteristics of the liquid crystal layers are changed by the voltages applied depending on the written images. In other words, when a linearly polarized light component having a predetermined polarizing direction enters as a light component to be read, the reflected light becomes elliptically polarized light. Therefore, parts of the S-polarized light components are converted into P-polarized light components by the reflecting light valves 22, 23, 24, and reenter the polarization beam splitters 19, 20, 21. The P-polarized light components included in the reflected light pass through the polarization beam splitters 19, 20, 21, and are synthesized into a single component by a color synthesizing optical system comprising dichroic mirrors 31, 32 and a plane mirror 33. The component enters a projection lens 34. The S-polarized light components are reflected by the polarization beam splitters 19, 20, 21 and travel toward the light source 11. In this way, the optical images formed depending on the changes in the double refraction characteristics of the liquid crystal layers of the reflecting light valves 22, 23, 24 are magnified and projected on a screen (not shown).
In the structure shown in FIG. 2, a single projection lens, namely, the projection lens 34, is used. This structure is more advantageous than that including three projection lenses in the convergence adjustment and color uniformity of projected images and the compactness of the apparatus. However, this structure requires a color synthesizing optical system to synthesize reflected light components from the three light valves into a single light beam. The color synthesizing planes of the dichroic mirrors 31, 32 to be used in this case are inclined with respect to the optical axis of the projection lens 34. When inclined parallel plane plates (dichroic mirrors) having a specific thickness are disposed in the optical path of the image-forming optical system, astigmatism occurs, thereby significantly deteriorating the resolution of projected images.
It is conceivable that the following two methods are used to reduce or eliminate astigmatism generated at the dichroic mirrors 31, 32.
a) The substrates of the dichroic mirrors are made thinner. PA0 b) Dichroic prisms which do not generate astigmatism are used. PA0 image forming means for forming optical images; PA0 second lens means for transmitting light from said image forming means; PA0 plane plate members for transmitting and/or reflecting light from said second lens means; and PA0 first lens means for receiving light from said plane plate members and delivering processed light, wherein PA0 image forming means for forming optical images; PA0 polarizing and splitting means having a polarizing and selecting characteristic so as to reflect said first polarized light component and transmit said second polarized light component, said first and second polarized light components being perpendicular to each other; PA0 plane plate members for transmitting and/or reflecting light; and PA0 a lens means for receiving light from said plane plate members and delivering processed light, in this sequence, wherein PA0 said plane plate members are disposed obliquely with respect to an optical axis of said lens means, PA0 said polarizing and splitting means are equipped with transparent substrate having a predetermined thickness, also having a film with said polarizing and selecting characteristic and disposed obliquely with respect to the optical axis of said lens means, PA0 the plane including a normal line to said transmitting and/or reflecting plane of said plane plate members and an optical axis of said lens means is perpendicular to a plane including a normal line to said film-formed plane of said transparent substrate and the optical axis of said lens means. PA0 image forming means for forming optical images; PA0 second lens means for transmitting light from said image forming means; PA0 plane plate members for transmitting and/or reflecting light from said second lens means; and PA0 first lens means for receiving light from said plane plate members and delivering processed light, said plane plate members being disposed obliquely with respect to an optical axes of said first lens means, wherein PA0 said first lens means receive light from said plane plate members and project said optical images, PA0 said second lens means is disposed between said plane plate member and said image forming means, PA0 a magnification of said second lens is adjusted so that a magnitude of an astigmatism generated depending on said plane plate members is smaller than the magnitude generated when said second lens means are not used. PA0 image forming means for forming optical images; PA0 polarizing and splitting means having a predetermined thickness and transmitting or reflecting light depending on the polarized light component, PA0 plane plate members having a predetermined thickness and transmitting and/or reflecting light, PA0 a lens means for receiving light from said plane plate members and delivering processed light, in this sequence, said plane plate member being disposed obliquely with respect to an optical axis of said lens means, wherein PA0 a plane including a normal line to said plane plate members and the optical axis of said lens means is perpendicular to a plane including a normal line to said polarizing and splitting means and the optical axis of said lens means, PA0 a relationship between a thickness of said plane plate member and a thickness of said transparent substrate is adjusted so that an astigmatism generated at said plane plate member and an astigmatism generated at said polarizing and splitting means can be compensated for with each other. PA0 a plurality of image forming means for forming optical images depending on the change in double refraction; PA0 polarizing and splitting means provided at each of said image forming means and for splitting polarized light components whose polarized wave planes are perpendicular to each other; PA0 second lens provided at each of said polarizing and splitting means and for transmitting light from the polarizing and splitting means; PA0 color synthesizing means for synthesizing light beams from each one of said polarizing and splitting PA0 means into a single light beam; and PA0 the first lens for receiving light from said color synthesizing means and delivering processed light, in this sequence, wherein PA0 said polarizing and splitting means have a transparent substrate having parallel planes and disposed obliquely with respect to an optical axis of said first lens, and a dielectric multilayer film having a polarizing and selecting characteristic is formed on the transparent substrate, PA0 said color synthesizing means comprise a plurality of dichroic mirrors which are respectively made by forming a dielectric multilayer film having a wavelength selection characteristic on a transparent substrate having parallel planes and disposed obliquely with respect to the optical axis of said first lens, PA0 a plane including a normal line of a dielectric multilayer film formation plane of each one of said dichroic mirrors and an optical axes of said first and/or second lens is perpendicular to a plane including a normal line of a dielectric multilayer formation plane of said polarizing and splitting means and the optical axes of said first and/or second lens. PA0 a light source delivering light including the three primary color components; PA0 color separating means for separating the light from said light source to the three primary color components; PA0 three prepolarizers for receiving each one of three output light beams from said color separating means, respectively; PA0 polarized beam splitter, disposed at each of said prepolarizers, respectively, for transmitting or reflecting light beams from said prepolarizer; PA0 reflecting light valve, disposed at each of said polarized beam splitters, respectively, for forming optical images on the basis of light beams from a predetermined image source and an optical writing means; PA0 color synthesizing means for synthesizing light beams from said polarization beam splitters into a single light beam; PA0 second lens disposed between said polarized beam splitter and said color synthesizing means so as to correspond to said polarized beam splitter; and PA0 a first lens for receiving light from said color synthesizing means and delivering processed light, wherein PA0 said image source generates image light beams to be applied so as to correspond to said reflecting light valve, PA0 said light writing means form the images of said image light beams from said image source on said reflecting light valve, PA0 each one of said polarization beam splitters have polarizing and splitting mirror with a dielectric multilayer film having a polarizing and selecting characteristic, and formed on the transparent substrate thereof having parallel planes, said each one of polarizing light beam splitters is disposed between said reflecting light valve and said second lens, PA0 said color synthesizing means comprise two dichroic mirrors, an optical compensation plate, both planes of which are subjected to reflection prevention treatment, and a plane mirror, and said color synthesizing means is disposed between said first lens and said second lens, PA0 a polarizing and splitting plane of said polarizing and splitting mirror, color synthesizing planes of said two dichroic mirrors and a reflection prevention plane of said optical compensation plate are disposed obliquely with respect to an optical axis of said first lens, respectively, and PA0 a plane including a normal line of the polarizing and splitting plane of said polarizing and splitting mirror and an optical axis of said first lens and/or said second lens is perpendicular to a plane including a normal line of the color synthesizing plane of said two dichroic mirrors, a normal line of the reflection prevention plane of said optical compensation plate and the optical axis of said first lens and/or said second lens. PA0 a light source delivering light including three primary color components; PA0 color separating means for separating the light from said light source into the three primary color components; PA0 three light valves for receiving three each one of output light beams from said color separating means, respectively; PA0 color synthesizing means for synthesizing output light beams from said light valves into a single light beam; PA0 a second lens disposed between said light valves and said color synthesizing means; and PA0 a first lens for receiving light from said color synthesizing means and delivering processed light, wherein PA0 said color synthesizing means comprise a first dichroic mirror, a second dichroic mirror and a plane mirror, and PA0 said second lens is disposed between said first dichroic mirror and said second dichroic mirror.
Since the color synthesizing optical system is disposed between the light valves and the projection lens, the system must be considered as part of the projecting optical system. In this case, in particular, the reflecting planes of the dichroic mirrors are required to have high plane accuracy. It is difficult to attain high accuracy as the substrate is made thinner. Particularly when high-precision projected images, such as images for high-definition TV, are displayed, the substrate must have a thickness of at least 1.5 mm or more to satisfy the plane accuracy required for the reflecting planes of the dichroic mirrors. For this reason, the method of a) has a limit, and it is thus difficult to reduce the adverse effect of astigmatism on the resolution.
The method b) is carried out in two approaches: one approach wherein a color synthesizing plane is formed on the junction plane of two glass prisms used as dichroic prisms, and the other approach wherein a liquid having the same refractive index as that of the substrate of the dichroic mirror is filled in a transparent container including a dichroic mirror so as to function as a prism as a whole. However, the former approach requires high cost and causes heavy weight. The latter approach requires a very long optical path for the liquid in the color synthesizing optical path, whereby the focus drift of projected images due to the change in refractive index depending on temperature becomes not negligible. Because of these reasons, it is difficult to adopt these approaches.